The Sigma offers a better value over the Canon, but not by much. The Canon is sharper, but has the push/pull zoom. The Sigma gives you a little more zoom range and a twist zoom, but is less sharp due to the higher zoom range. The Canon runs just over $1400, and the Sigma runs just under $1000. Both great lenses, which one suits you best, only you can answer that.

70-200 f2.8IS is also nice but using 2x on that won't match 100-400L. So if you need 400, then get 100-400. If you need low light, then 70-200 f2.8 IS.

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Again, I am new to all of this so if you don't mind please explain why the at 400mm the 70-200 f2.8 with a 2x teleconverter will not perform as well as the 100-400 lens?I would have thought it woulddo as wellsince itis a 2.8.

How does the Sigma 80-400 stack up to the Canon 100-400? Similar price range?

2xTC on a zoom is no match (picture quality wise) for another lens of twice the focal lenth even though the end aperture would be same (f5.6 in this case). If you were talking about using 2xTC on lens like canon's 300 f2.8 IS, then itis different. Even in that case, a 600 f5.6 (nobody makes this lens) would be much better.

If you need range, yougo for it.

REgarding80-400OS, it is slow focussing.If you don't need IS, then you should look at Canon 400mm f5.6 prime.Cheapest of all is Sigma 50-500.